To provide an external combustion engine for improving heat recovery efficiency.

An external combustion engine 1 having an evaporator 2 and a condenser 3 is switched among a communication state for communicating a gas side of the evaporator 2 and a gas side of a cylinder 4b by a gas side rotary valve unit 5, a communication state for communicating a gas side of the condenser 3 and the gas side of the cylinder 4b, and a communication state for communicating the gas side of the evaporator 2, the gas side of the condenser 3 and the gas side of the cylinder 4b, and is switched between a communication state for communicating a liquid side of the condenser 3 and a liquid side of the cylinder 4b by a liquid side rotary valve unit 6, a communication state for communicating a liquid side of the evaporator 2 and the liquid side of the cylinder 4b, and a communication state for communicating the liquid side of the evaporator 2, the liquid side of the condenser 3 and the liquid side of the cylinder 4b, so as to reciprocate a piston 4a using high-pressure gas of the evaporator 2 and low-pressure gas of the condenser 3.